Various production processes require that a work operation be performed on articles as they are moved through a work station. Because the articles are moving and because of inherent electrical and mechanical delays, the actuation signal which commands the apparatus to perform the work operation must be given before the article reaches the work station. This lead time assures that the work operation will be performed when the article is at the work station.
One example of a process where these factors are applicable is label application. In label application, articles to be labeled are continuously conveyed past a label applicator. The label applicator applies a label to each of the articles as the article is moved through a labeling station. Although different arrangements are possible, typically, the label applicator releasably retains a label at a first location. When the article to be labeled nears the labeling station, an actuation signal is provided commanding that the label applicator immediately transfer the label from the first location to the article. In this specific example, the actuation period is the period of time from the actuation signal until the adhesive face of the label contacts the article.
One problem which occurs in processes of this kind is that line speed, i.e., the speed of article movement, is a variable. Thus, prior art systems that provide the actuation signal a fixed distance from the work station introduce the risk that the work operation will not be performed on the article at all, or if it is, it will be performed at the wrong location. For example, in the case of label application, the label may be applied to the wrong location on the article or it may miss the article entirely.
This problem can be overcome with a velocity compensator. Prior art velocity compensators are generally complicated and very expensive. Moreover, analog prior art systems are subject to drift.